The invention relates to a remote power feeding and protection system and a remote power feeding extractor protection device for equipment connected by two transformers to a four-wire transmission link ensuring remote power-feeding.
It particularly concerns remote power-fed equipment provided with a pulse transmission transformer and a pulse receiving transformer which are each respectively connected to one of the two pairs of wires of a four-wire transmission link by which the equipment in question is also partially or entirely D.C. remote power-fed. This applies, for example, as regards linking equipment between terminals--or terminal adaptors--and private unit automatic exchanges--or local networks--which are defined as S-interface equipment in the networks integrating telephony and data currently designated by the latter symbols.
It is normal practice in telephony to provide line equipment of electronic networks, both temporal and spatial, with translators enabling continuity of the link to be interrupted between a two-wire transmission and feed line and the switching exchange for direct-currents, whilst ensuring the passage of currents at voice frequencies. Normally, such translators comprise a transformer with two windings, known as primary windings, and one winding, known as a secondary winding, to which associated a capacitor inserted between is two primary windings.
The capacitor constitutes a short-circuit for voice frequencies transmitted, either from the two serial primary windings towards the secondary winding, or inversely.
On the other hand, this capacitor prohibits the flow of the D.C. power-feed current which the line supplies as regards the two primary winding. A remote power-feeding extraction arrangement makes it possible to selectively pick up the direct current which the line supplies for the requirements of the equipment fed by this line.
The capacitor and the two primary windings of the transformer operate with continuous polarizations much greater than the amplitudes of voice frequency signals, which is unsuitable for transmission of the latter.
The volume of the primary windings has to be substantial owing to the low resistance admissible for these windings and the high continuous polarization of the magnetic circuit.
It is an additional common practice to place on the line current limiting resistors which are often associated with resistors with a positive temperature coefficient so as to limit the direct currents to reasonable values.
Such arrangements provided for voice frequency signals are no longer acceptable for the transmission of data in a pulse form with rates capable, for example, of reaching 96 Kbit/s.
It is therefore vital to eliminate line resistors. To compensate for the elimination of these resistors, it is then necessary to dispose of a protection device making it possible to avoid heating the transformers and restrict interference for the equipment fed by this transformer in the even of a short-circuit or inversion of wires, or in the even of injecting low-range industrial voltages.
Moreover, it is sought to provide links which allow for simultaneous bidirectional transmissions, which accordingly involves one pair of wires per channel and one transformer per pair of wires in the equipment in question.